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Philosophy behind “Colony size and foraging range in seabirds”

Basic questions in animal collective behaviour are still unanswered. Bird coloniality is a good example: Why do many bird species breed in colonies? Why is colony size so variable within and between species? Why is breeding in colonies often synchronised? Historically, ultimate explanations at the group or species level have been considered to explain why bird groups, populations and species behave in a certain way. For instance, it has been hypothesised that fledgling predation dilution is the ultimate cause for breeding synchrony in bird colonies. However, this "ultimate cause" approach often fails because they don't explain why these collective patterns are created in the first place.

In the Jovani lab http://jovanilab.com/ we are exploring a different path; we approach these questions from the understanding that we are dealing with complex systems where the collective patterns we observe (e.g. colony size, breeding synchrony) emerge from the (adaptive) behaviour of individuals interacting among them and with the (biotic and abiotic) environment. Given that we are studying huge populations of highly unrelated individuals (i.e. these are not eusocial species like bees or ants) our major goal is the identification of the relevant adaptive individual behaviour/s that best explains the emergence of these collective patterns. This implies at least two steps in our research (in other studies we also use an individual-based modelling approach). Here, we exemplify these steps using the trilogy that concludes our recently published paper in Oikos, “Colony size and foraging range in seabirds”, in which we try to explain why colony size differs widely between seabird species:

1. Quantitatively describe the pattern.

Believe it or not, despite a strong body of research on bird colony size variation, we still have a poor understanding of the shape of this variation. In two previous papers analysing thousands of colonies of varying sizes for tens of species, we showed two things: (A) There is huge variation in colony size within and between seabird species, with colony size frequency distributions from log-normal to power laws, often spanning from very small to very large colony sizes within species (Jovani et al. 2008). (B) This intraspecific variation does not blur interspecific differences, and some species consistently show much larger medians, 95th percentiles, and maximum colony sizes than other species. We found this by comparing colony sizes of same seabird species in different geographic areas (Jovani et al. 2012). That is, typical and maximum colony sizes are species-specific traits despite high intraspecific variation in colony sizes.

One can propose an ultimate explanation for why colonies should be of a certain size for a given species. However, this will never explain why these colony sizes arise in the first place. For this reason, our approach was to search for “how” explanations of colony size. Specifically, we tested an old hypothesis called the Ashmole’s halo (Ashmole 1963) that suggests that seabird populations are regulated by food abundance around colonies. This is because seabirds are central-place foragers that need to go back and forth to the sea to gather food for their nestlings. Our hypothesis was based on the simplest model: the equation of the area of a circle (area = π * radius2). In other words: foraging area = π * foraging range 2. All else being equal, doubling the foraging area doubles the potential food gathered and thus, the potential colony size. Therefore, we predicted a correlation between foraging range and colony size with a slope of 2 in a log-log plot. After a thorough review of literature on foraging distances and gathering a huge dataset on colony sizes (please see the acknowledgements in the paper), we correlated the maximum foraging ranges with the colony sizes reported for 43 seabird species in the Northern Hemisphere. As expected, we found that foraging range imposes a constraint on the maximum colony sizes seabirds can achieve; and this ceiling was imposed by a slope of 2 (we found that through a quantile regression approach). Thus, our results strongly support the hypothesis that the huge variation in maximum colony sizes between species is the consequence of species having different abilities to forage up to different distances from their colonies. Note that our results only make sense if food is depleted (or prey escape) around colonies. Otherwise, species with short foraging ranges would have the largest colonies given the unlimited availably of food close to their nests. Thus, our study strongly supports the Ashmole idea (in contrast to his PhD supervisor David Lack, who hypothesised that seabird populations would be regulated by bird winter survival) that seabird colony sizes (and likely population sizes) are strongly regulated by food availability.

This general approach to collective behaviour in vertebrates has been previously used in other papers in the Jovani lab to understand:

Collective foraging behaviour in vultures testing an IBM (with three different hypotheses) against empirical data on the number of vultures attending carcasses in the field (Cortés-Avizanda et al. 2014).

Other collective behaviours studied in the Jovani lab:

A first instance of fractal nest distribution in a bird species (Jovani and Tella 2007).

Why species colony sizes and the colony sizes experienced by individuals are not the same thing (Jovani and Mavor 2011).

Oikos Journal

Oikos is a journal issued by the Nordic Ecological Society and is one of the leading peer-reviewed journals in ecology. Oikos publishes original and innovative research on all aspects of ecology, defined as organism-environment interactions.